(1) Field of the Invention
This invention relates to a polarity switching circuit. More specifically, the present invention relates to a voltage polarity switching circuit, capable of selectively producing positive and negative output voltages whose absolute values are equal to each other, which circuit is suited for fabrication as a semiconductor integrated circuit.
(2) Description of the Prior Art
Positive and negative reference voltages or reference currents having absolute values which are equal to each other are required in a so-called A/D converter for converting analog electric signals of positive and negative polarities into digital codes, and such reference voltages or currents are required in a so-called D/A converter for making the reverse conversion. In order to fabricate the A/D or D/A converter in a small size and with a high accuracy by means of a semiconductor or monolithic IC, however, it is not advantageous from the aspects of both fabrication technique and fabrication economy to incorporate both positive and negative reference voltage sources in the converter. Accordingly, it is usually the practice to incorporate only either the positive or the negative reference voltage source and to combine it with a voltage polarity switching circuit for selectively changing over the polarity of the input voltage, which is the above-mentioned reference source, so as to obtain reference voltages of both positive and negative polarities.
However, since the conventional circuit construction of this kind is affected by the stray capacitance occurring in the integrated circuit, the absolute values of the two positive and negative voltages cannot be made equal to each other, as will be explained later in further detail. To approximate these two voltage values as closely as possible, the charging capacitor must have large electrostatic capacitance and a large area must be secured on a semiconductor substrate to form such a capacitor.